1. of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems which include a plurality of elevator cars under the control of a system processor.
2. Description of the Prior Art
When a building requires more than one elevator car to serve the traffic, some sort of supervisory control means is usually provided in order to insure efficient elevator service. For example, in U.S. Pat. No. 2,695,077, which is assigned to the same assignee as the present application, the elevator cars are dispatched successively from a dispatching floor by a main dispatching device. Failure of the main dispatching device would terminate all elevator service once all of the elevator cars have returned to the main dispatching floor. Thus, this patent discloses the use of an emergency dispatching device, in order to continue elevator service.
U.S. Pat. No. 3,854,554, which is assigned to the same assignee as the present application, discloses an improved supervisory system control arrangement for a plurality of elevator cars, in which the cars are controlled by inhibit or overriding signals, rather than by direct commands. The elevator cars each include a car controller which enables the associated elevator car to independently respond to a registered hall call. The supervisory system control decides which elevator car should answer a specific hall call and issues signals which inhibit the other elevator cars from responding to the call. Failure of the supervisory control in a mode in which inhibit signals are not sent to the cars does not terminate elevator service, and it does not require a standby emergency dispatcher, as all elevator cars are automatically on independent control in the absence of inhibit signals.
Failure of the supervisory control in a mode which may continuously provide inhibit signals, or otherwise adversely affect the ability of the elevator cars to operate properly, is detected by monitoring a selected function of the supervisory control. For example, when the supervisory system control includes a digital computer with the operating strategy stored in the memory thereof in the form of a program, the stored program must be run repeatedly to continuously update the systems. U.S. Pat. No. 3,854,554 suggests a hardwired timing circuit, as opposed to a software timing circuit, whose output is held high by periodic accessing by the stored operating program. Failure of the supervisory control to access the timing circuit at the proper frequency allows it to time out and provide a low signal which is used to block any signals which may be provided by the supervisory control from being considered by the car controllers of the various elevator cars.